1. Field of the Invention
The present invention relates to a pattern-forming method, and to a radiation resist for use when working this pattern-forming method.
2. Description of the Related Art
Due to the recent increase of the degree of integration of semiconductor integrated circuits, it has become difficult to uniformly cover with a resist undulations formed at the multi-layer wiring operation, and accordingly, the utilization of the multi-layer resist method using a flattening layer as the lower layer has been attempted. To practically work this method, however, a resist having an excellent resistance to oxygen plasma etching, a high sensitivity, and an excellent resolving power must be developed.
Nevertheless, a positive resist having a practically sufficient performance as the upper layer resist of a two-layer resist for use in the electron beam lithography has not been developed although, as a positive resist having a good resistance to oxygen plasma etching, there have been proposed a poly[trimethylsilylmethyl(.alpha.-chloro)methacrylate] having a structure represented by the following formula (3): ##STR2##
wherein Me represents --CH.sub.3 ; the same will apply hereinafter,
and poly(trimethylsilylmethyl methacrylate) represented by the following formula (4): ##STR3## (see, for example, T. Tada and T. Ushirogouchi, Solid State Technology, June 1989, pages 91 through 95). In the case of the former resist, a high-sensitivity positive pattern can be obtained in a region of small irradiation doses by electron beam exposure, but at an irradiation dose larger than about 100 .mu.C/cm.sup.2, a negative pattern is formed, and accordingly, the region wherein the compound can be used as the positive resist is very narrow. The latter resist does not have a stable resolution sufficient for the resolving of a sub-micron pattern.
Accordingly, the present inventors investigated a copolymer of trimethylsilylmethyl methacrylate with
2,2,2-trifluoroethyl(.alpha.-trifluoromethyl) acrylate, which is represented by the following formula (5): ##STR4## and a composition formed by adding an azide compound to this copolymer, to improve the resolution. Although these resists can resolve a sub-micron pattern, the sharpness of the pattern is relatively poor and the resistance to oxygen plasma etching is only slightly higher than the practically allowable lower limit (the selection ratio to the lower layer is 20).
Accordingly, if the polymer of formula (3) is used as the upper layer resist, there is a danger that a negative pattern will be formed in a beam-doubled portion at the electron beam exposing, and if the polymer of formula (4) or (5) is used, the sharpness of the pattern is poor and the transfer of the pattern to the lower layer is not complete.